Sealing and Containment
Fines generated from bulk products are only a problem when they become airborne and/or settle in large amounts around the process plant. The design of plant should be examined carefully to minimise dust release to atmosphere and to prevent spillage. It is important where practical to:
- Not allow product to fall freely from the head of a stocking out conveyor for example particularly when the process is outside and falling from a considerable height.
- Not allow product to transfer down open chutes particularly where there is more than 1 metre elevation involved.
- Not allow product to become too dry. This increases dust release.
- Arrange product to be screened in a logical manner so that the fines are taken out first as opposed to last.
- Not allow product to free-fall into trucks or rail wagons particularly where this is carried out in the open and subject to windage.
The above are some simple examples of good plant design from a dust control point of view.
When considering sealing and containment some examples are:
- Conveyors should normally be enclosed where they are outside and subject to windage.
- Feed chutes should be sealed into skirted area of receiving conveyor (skirting at least 6 times the conveyor belt width).
- Total process plant should be installed inside buildings where possible to reduce windage effect and to contain as much of the dust as possible to the interior of the buildings.
- Install the most effective belt scrapers where there is a tendency for fine product to return on the understrand of the conveyor or where the product has a high moisture content. Conveyor belts can be responsible for shedding a considerable amount of dust into the atmosphere adjacent to each return idler and also attribute to spillage. If possible the manufacturers should be used to service belt scrapers on a regular basis.
- Stocking out conveyors should be of the luffing type where possible so as to reduce the free fall of the product. Alternatively the stockpile should be maintained as high as possible - again to reduce the free fall of product particularly when exposed to high wind conditions. There are various chutes available, for example, the Didcot chute where this runs vertically from the conveyor head to the ground and has a number of staggered openings sealed by self-opening neoprene sheeting. The product falls down the chute and out through the appropriate openings depending on the height of the stockpile. This will reduce the open fall of materials in free air. (Not to be used where product is not free flowing).
- Screens should be encapsulated and sealed into the underpan where possible.
- Flooring in screen houses should normally be solid and sealed into the screen underpans to prevent the release of dust from the bins into the main screening area.
There are many other examples of sealing and containment depending on the process plant and the nature of material being handled.
Although sealing and containment will improve the release of dust to atmosphere and may be regarded as good design practice there are many cases where this is not adequate and further dust control measures have to be considered. These dust control measures are often demanded by the environmental agencies as a condition of planning consent.